1. Field of the Invention
This invention pertains to the field of lubricants, especially greases.
2. DESCRIPTION OF THE PRIOR ART
It is desirable in the selection of greases to consider the environment that the grease will have to operate under and in many cases to select a grease which has properties which will allow it to perform adequately where two surfaces come together with considerable force. A lubricant between these surfaces must possess a property known as Extreme Pressure (EP) Tolerance. This property allows the lubricant to continue to lubricate the two surfaces and to stay between the two surfaces even though the surfaces come together with considerable force making it very difficult to keep a lubricant in place and to keep the two surfaces from actually touching each other. The art has many references to additives which impart extreme pressure properties to greases, that is the extreme pressure properties of the resulting grease having the combination of additives as disclosed by applicants have extreme pressure properties superior to greases containing either component of the proposed additive combination. In general, the additive combination of applicants' invention consists of low density polyethylene and a conventional extreme pressure additive. The low density polyethylene is melted into the grease and the combination imparts a synergistic effect to the extreme pressure properties of the resulting grease. Polyethylene has been used and is disclosed in the art for other purposes. U.S. Pat. No. 3,432,431 discloses a grease having improved endurance properties, which is not related to extreme pressure properties, wherein a high density polyethylene is cold dispersed, not melted, into a grease. Extreme pressure properties are not discussed. In fact the high density type of polyethylene used is not suitable for applicants' invention. The only mention of extreme pressure properties comes in a general listing of other additives which the patentee says may be used in his grease. One of these additives, molybdenum disulfide, is known to be used as an extreme pressure additive. However, the teachings of this patent do not discuss extreme pressure properties and the reference to molybdenum disulfide is only used in a general listing of other additives. Also, patentee uses high density polyethylene which is cold dispersed unlike the low density polyethylene which is melted, hot dispersed, in applicants' invention.
U.S. Pat. No. 3,753,906 discloses a brake cylinder grease which by the definition of the numerated properties does not have extreme pressure properties. In this patent a medium density polyethylene is melted into grease to thicken it. The only reference to extreme pressure properties is in a general and broad listing of other additives wherein the term "extreme pressure agents" appears. However, no details are given in this instance and no teaching is given to indicate that any extreme pressure properties are needed or desired in the grease disclosed and claimed, nor is there any teaching of synergism at all. In fact, extreme pressure properties are not even measured in the grease which is claimed. The general thrust of this patent is in the order of addition of the polyethylene and the clay and the effect of the order of addition on the endurance test which provides the improvement of the grease. Further, medium density polyethylene is required whereas low density polyethylene is required for the present invention.
Applicants on the other hand, have as an object the production of a grease having synergistic mixture of additives which impart extreme pressure agent. The low density polyethylene is melted into the grease.